Referring to FIGS. 1 and 2, a conventional fluid driven electric generator as disclosed in Taiwanese Utility Model No. M502286 includes a base unit 9, a driving unit 8 and at least one power generating unit 7. The base unit 9 includes a shaft 91. The driving unit 8 includes a tubular member 81 surrounding the shaft 91, and a plurality of blade modules 82 disposed on the tubular member 81. The power generating unit 7 includes a stator 72 fixed on the shaft 91, and a rotor 71 coupled with the tubular member 81. The blade modules 82 are driven by a fluid flow to rotate the rotor 71 relative to the stator 72 so as to produce an induced electric current. Since the rotational speed of the rotor 71 relative to the stator 72 is limited depending on the fluid flow, it is difficult to enhance the power generating efficiency. In other words, it is required to increase amounts of coil windings of the power generating unit 7 in producing an expected electricity quantity, which results in a relatively high material cost.